This invention relates to a cold mirror, and more particularly to an all-polymeric cold mirror which reflects light in the visible wavelength region while transmitting a substantial portion of light in the infrared wavelength region, and which can be designed either to reflect, transmit, or absorb light in the ultraviolet region of the spectrum.
In the production of lighting systems for medical treatment, window displays, theatrical lighting, and other applications, a problem exists from the harmful effects of infrared or ultraviolet light on illuminated objects. Light sources which emit infrared radiation can cause heating of illuminated objects, which is often undesirable and damaging. For example, certain jewels such as rubies and pearls have a substantial water content. Such jewels lose their luster if moisture is lost because of infrared heating from lights used to illuminate the jewels.
Light sources which emit ultraviolet radiation can also be damaging to illuminated objects. For example, it has been found that ultraviolet radiation is a contributing factor to the fading of colors in oil paintings and tapestries displayed in museums and galleries. General merchandise displays in retail outlets may also be subject to damage from ultraviolet radiation. Further, the combination of ultraviolet and infrared radiation from light sources may cause even more rapid degradation of illuminated objects.
To combat this problem, cold mirrors, also referred to as cold light mirrors, have been developed which reflect, visible wavelengths of light, but transmit longer-wave infrared or shorter-wave ultraviolet radiation. The cold mirrors are arranged so that visible light from a light source is reflected onto an object to be illuminated, while infrared radiation is transmitted through the mirror and away from the object. This minimizes heating and potential damage to the illuminated objects. Cold mirrors are currently widely used in lamps for projectors, for lighting in studios and theaters, art displays, shop windows, and in security and medical applications.
Typically, cold mirrors comprise an uneven number of absorption-free layers of high and low refracting dielectric materials applied alternately to a glass substrate. Zinc sulfide and magnesium fluoride are two commonly-used dielectric materials as well as titanium oxide and silicon dioxide. A typical method of production of such cold mirrors is high vacuum deposition.
For example, Great Britain Patent No. 1,262,163 teaches a cold mirror used in cinema projectors which is formed by the vacuum deposition of interference layers on a glass substrate. The substrate is permeable to heat radiation and comprises varying alternating layers of silicon, silicon oxide, and magnesium fluoride and titanium oxide.
An alternative to glass has been the use of a metal substrate as taught in McLintic, U.S. Pat. No. 3,944,320, which teaches a cold mirror comprising a metal substrate coated with first and second pigmented vitreous coatings and a dielectric interference coating. However, the mirror requires the addition of a black vitreous enamel which is capable of absorbing infrared radiation as the metal will reflect infrared radiation.
Doctoroff et al, U.S. Pat. No. 3,645,601 also teaches a reflector comprising an aluminum substrate including a dielectric interference coating which reflects visible wavelengths of light and absorbs or diffuses at infrared wavelengths.
However, a major disadvantage of prior art cold mirrors is that they require deposition of dielectric materials in multiple separate processing steps using relatively expensive and time consuming vacuum deposition techniques. In addition, special care must be exercised to ensure uniformity of film thickness over the entire surface of each individual substrate. Also, once deposited, the coatings and the substrates to which they are adhered cannot be further shaped or formed. Further, the coatings are subject to chipping, scratching, and/or corrosion and must be protected. All of these factors add to the expense of production. The need to deposit the layers on a glass substrate increases the thickness and weight of the final product. Because vacuum deposition techniques must be used, it is difficult and expensive to fabricate coatings which cover large surface areas; consequently, because of that difficulty and expense, many cold mirrors are relatively small. Finally, vacuum deposition cannot be used with parts having certain geometries; for example, the interior of a tube or deep cavity cannot be coated using standard vacuum deposition techniques.
Another type of cold mirror utilizes dichroic (multilayer interference film) coatings which are typically utilized on the reflectors of tungsten or halogen lamps. Such dichroic cold mirrors comprise a glass surface coated with a metal film that reflects visible light while transmitting infrared as well as absorbing ultraviolet light. For example, Levin et al U.S. Pat. No. 4,604,680 teaches an infrared floodlight for security applications which uses a tungsten halogen light source and dichroic hot and cold mirrors comprising a glass substrate having multiple layers of titanium dioxide and silicon dioxide coated thereon for directing infrared radiation toward the assembly's lens. Such dichroic reflectors have been widely used in halogen lamps for museum displays to prevent degradation of works of art from prolonged exposure to ultraviolet radiation.
Lawson, U.S. Pat. No. 4,380,794, teaches a cold mirror for surgical applications comprising a high-temperature resistant polyetherimide which has vacuum-deposited thereon a dichroic coating. The high-temperature resistant polymer is used so that it will be able to withstand temperatures encountered during vacuum deposition to the dichroic coating.
Other halogen lamps produce a cooler light by a visibly transmissive, infrared reflective coating for the inner filament tube that reflects infrared radiation back toward the filament. The infrared radiation is used to maintain filament temperature and produce more visible light. However, such lamps are very expensive to produce.
Multilayer polymeric reflective films are known. Alfrey, Jr. et al, U.S. Pat. No. 3,711,176 teaches a multilayered highly reflective thermoplastic body fabricated using thin film techniques. That is, the reflective thin film layers of Alfrey, Jr. relied on the constructive interference of light to produce reflected visible, ultraviolet, or infrared portions of the electromagnetic spectrum. Such reflective thin films have found use in decorative items because of the iridescent reflective qualities of the film.
Other multilayer reflective bodies are also taught in the art. See commonly-assigned Wheatley et al, U.S. Pat. No. 5,122,905, Wheatley, U.S. Pat. No. 5,122,906, and Wheatley et al, U.S. Pat. No. 5,126,880. However, these reflective bodies are designed to be uniformly reflective over substantially the entire visible range as well as reflecting in the infrared range.
Accordingly, the need still exists in the art for cold mirrors which are inexpensive and easy to produce, and which reflect visible wavelengths of light while transmitting infrared wavelengths. The need also exists for cold mirrors which can be formed and/or bent into complex shapes, cold mirrors which are free-standing and require no glass or metal substrate for support, and cold mirrors which can be laminated to a variety of other substrates. The need also exists for cold mirrors which can be designed to reflect or absorb at ultraviolet wavelengths.